Rapid advances have been made in improving the resolution and depth of focus of images formed using photolithographic processes in recent years. These developments have been particularly useful in forming images on wafer type substrates as are used in the fabrication of integrated circuits.
There are several major steps which are common to all integrated circuit imaging processes used to replicate integrated circuit pattern geometries on the wafer surfaces. First the substrate, preferably a wafer, is coated with a photoresist which is usually composed of a phenolic resin known as a Novolak, and a photoactive compound or PAC which makes the resist sensitive to a particular exposure energy such as light, e-beams and X-rays. A photoresist composition, the photoresist and a solvent which dissolves the other components, is dispensed onto the surface of the substrate and is formed into a uniform thin layer of resist across the surface of the substrate, preferably by spinning Thereafter the coated substrate is preferably heated for a short period of time at a low temperature to remove the solvent, a process generally called soft baking. This baking is frequently carried out at a temperature in the neighborhood of 90.degree. C. Thereafter a suitable mask representing the image to be formed in the resist is arranged on or in connection with the coated substrate. The image on the mask is transmitted into the resist by exposure to a suitable source of energy such as light, X-rays or e-beams, depending on the resist used. For example, a chrome relief replica of the desired image can be used which is illuminated by exposing radiation and pattern transfer can be accomplished by contact between the mask and the substrate, proximity printing or projection printing.
The minimum width of the lines and spaces formed in the photoresist determine the resolution. This is influenced by, among other things, the wavelength of the exposing energy. The most commonly used lines of the mercury spectrum are the g-line (436 nm), h-line (405 nm) and i-line (365 nm). Generally, the use of shorter wavelengths results in better resolution and for a given resolution, in a greater depth of focus
In some techniques, the formation of standing waves within the resist is prevented by using a supplemental bake following the exposure to energy and prior to development. This so-called post exposure bake (PEB) is usually carried out at temperatures between 100.degree. and 120.degree. C.
Common to all processes is that the exposed resist is developed in an alkaline solution called a developer which results in the removal of selected, predetermined, areas of the resist Depending on the system and mask employed, the image on the resist can be positive or negative.
The verticality of the developed resist profile impacts the minimum resolution available, that is, the minimum feature sizes which can be delineated. Any scattered light, for instance light scattered within the resist film, results in unwanted erosion from the resist profile during development since those partially exposed areas are slightly attacked by the developer solution. This results in a degradation of the minimum resolution which can be achieved.
A number of techniques have been developed in the prior art to reduce this effect The most successful reported to date are known as the HARD and LENOS processes. Endo et. al., "High Aspect-Ratio Resist Pattern Fabrication by Alkaline Surface Treatment", Digest of Papers 1st MicroProcess Conference, Tokyo, page 164 (1988); Ogawa et. al., "LENOS: Latitude Enhancement Novel Single Layer Lithography", Digest of Papers 1st MicroProcess Conference, Tokyo, page 162 (1988). Both of these processes involve soaking the spun resist film in developer solution prior to exposure, whereupon trace quantities of the base are permitted to become absorbed into the surface. The application of baking cycles follow this treatment and are believe to catalytically enhance a crosslinking reaction between the binder resin and the unexposed photoactive component. As a consequence, an increased degree of surface inhibition is observed at the very top of the developing resist profile, which serves to improve the final profile. However, a disadvantage of these techniques is that the rather extended baking temperatures and times are needed, while only the top of the profile is improved, which is not surprising as the presence of the diffused developer is confined very much to the uppermost layer of the resist.
Moreau et. al., "Contrast and Sensitivity Enhancement of Resists for High Resolution", J. Vac. Sci. Technol B, Vol. 6, 2238, Nov/Dec 1988 have reported a slight improvement in resist profiles when an interrupted development cycle was carried out. Under these conditions, the wafer is rinsed and spun dry midway through development, as opposed to simply continuing with the second development cycle as in a conventional double puddle process. It has also been reported that this technique has been used with multiple interrupted development cycles, with further improvement. However this technique is not universally employable as it is not effective with some resists, most notably those such as KTi 895i where the resin is bound to the photoactive compound
There is therefore still a need for a process of general applicability where the time penalty is not as great. It is accordingly the object of this invention to provide such an improved process and to provide improved resist images resulting therefrom.